Amide inequivalence in the fibrillar assembly of islet amyloid polypeptide
Department of Molecular Biophysics and Biochemistry, Yale University, 260 Whitney Avenue, New Haven, CT 06520-8114, USA
1 To whom correspondence should be addressed. E-mail: andrew.miranker{at}yale.edu
Amyloid fibers are aggregated, yet highly ordered, β-sheet-rich assemblies of misfolded proteins. Order is established in such systems following profiles indicative of nucleation-dependent assembly. Nucleation dependence suggests that specific interactions, such as long-range contacts and/or strand registration, are critical to establishing initial fiber structure. Here, we show that amino acids at selected positions participate in key interactions that modulate the pathway of amyloid fiber formation by the hormone, islet amyloid polypeptide (IAPP). Specifically, we investigated the role of amide side-chain interactions in the process of IAPP assembly. We mutated five of the asparagine side chains in IAPP and assessed their effects on the kinetics of assembly. We find that the asparagine amide side chains strongly dictate the ability of IAPP to form fibers. In particular, the elimination of two specific asparagines results in near and total loss of amyloid, respectively. Interestingly, the two asparagines are located in a recently identified domain with 
-helical bias. These sensitivities are unusual for IAPP, as IAPP is generally tolerant to mutation. Here, we demonstrate this mutational tolerance by assessing 10 alterations at five distinct sites. In all cases, the constructs form fibers on timescales perturbed by less than a factor of two compared with wild-type protein. These findings indicate the presence of key specific interactions that are the determinants of IAPP amyloid formation.
Keywords: amylin/amyloid/IAPP/islet amyloid/type II diabetes
Received November 16, 2007; revised November 16, 2007; accepted November 19, 2007.